The effect of the differential thermal expansion is to cause at least the rotors, particularly the high pressure rotor, to bow upwardly. The amount of rotor bow is time dependent. For a given heat content within the gas turbine engine, the maximum rotor bow will occur some time after shut down, when the convective heat transfer has had time to act, but before the gas turbine engine has cooled down. The magnitude of the temperature differential between the upper portion and the lower portion of the gas turbine engine and the magnitude of the rotor bow depends on the heat content of the gas turbine engine, so that when the gas turbine engine has cooled down the temperature differential and rotor bow disappear.
The distortion, or bowing, of the rotor in itself is not harmful to the gas turbine engine. However, if it is desired to restart the gas turbine engine while the rotor of the gas turbine engine is distorted, or bowed, due to the differential thermal expansion the displacement of the centre of mass of the distorted rotor from the centre of rotation may create problems.
The first problem is large, damaging vibrations of the rotor and possibly rubbing of the rotor with the surrounding stator when the rotor passes through its first critical speed because the rotor of the gas turbine engine is distorted, or bowed, due to the differential thermal expansion. It is normal practice to arrange for the first critical speed of the rotor to be less than the idle speed. The rotor typically comprises two portions which are connected by a spigotted, bolted, joint. As the rotor cools down the spigotted, bolted, joint may become loose due to the differential thermal expansion and hence the vibrations of the rotor may produce wear at the spigotted, bolted, joints. The worn spigotted, bolted, joints exacerbate the vibrational response of the gas turbine engine rotor to rotor bowing.
The second problem is additional stresses are produced in the rotor when the rotor reaches high speed operation after start up if the rotor of the gas turbine engine is distorted, or bowed, due to the differential thermal expansion. The gas turbine engine may be started from a cooling condition and accelerated to idle speed and then to high speed before the rotor has warmed through to a uniform temperature circumferentially around the rotor. This is because of the high thermal inertia of the rotor discs and drums. The effect of the rotor bow is to superimpose an extra stress onto the high stress levels in the rotor, thus some circumferential parts of the rotor will have an additional tensile stress and some circumferential parts will have an additional compressive stress. The result is that the expected service life of a rotor that is frequently started in a bowed condition is less than that of a rotor that is never started in a bowed condition.
UK patent application GB2117842A discloses the use of ducts and blowers to circulate warmer gas from the upper portion of the gas turbine engine to the lower portion of the gas turbine engine or circulate cooler gas from the lower portion of the gas turbine engine to the upper portion of the gas turbine engine. This requires the provision of ducts and blowers to the gas turbine engine which adds weight and cost to the gas turbine engine.
UK patent application GB2117450A discloses the use of a particular mounting arrangement for the compressor casing and heaters to differentially heat the mounting to displace the casing to compensate for the distortion of the rotor. This requires the provision of the particular mounting and heaters which adds weight and cost to the gas turbine engine and does not solve the problem of vibration of the rotor.